here's a post i made on changing out your HVAC box
1. take it somewhere and have the freon vacumn out.
2. disconnect evap. core lines with handy A/C tool.
3. put a pan under the truck and pull rubber hoses off of the heater core.
4. next hop in the truck and remove the 5 or 6 screws by where the winsheild meets the dash.
5. remove the kick panels by the emergency brake pedal and the corrisponding one on the passenger side. theres a stud sticking out remove the nut. do the same with the passenger side.
6. remove cubby tray thing under radio.
7. remove the two nuts behind the cubby under the radio.
8. unbolt the steering column and just let it rest on the seat.
9. now would be a good time to go get the wife/girlfriend or a buddy who works for beer and have her/him stand on one side of the truck and you on the other. grab the dash assembly and rotate it back toward the seat.
8 1/2. side note you'll have to rotate/pull it off the studs that are behind that little cubby thing. also you will need keep an eye on all the wiring under the driver's side. you may need to disconnect a few things to get the dash back far enough.
9. remove the 5 or so bolts holding the HVAC box in. theres 1 or 2 going from the inside to outside. but the majority under the hood. you also have to remove the computer to get a few of the bolts.
10. disconnect vac. lines from HVAC box, label them so they go back where you got them.
11. pull HVAC box out replace heater core.
if possible go to junkyard and "practice" on a few junkers before you try it on your truck.
99, 4X4, stick, 2nd VP44, 6th carter, Vulcan liftpump relo. kit, Mech. fuel press. guage, BHAF, silencer ring MIA, DIY Rebuilt HX35W, 2nd str8 pipe, crack free junkyard dash for $25!, LSD clutch restack, GM wheel cylinders, got 210,000 miles, been mine for 10 years!